Monomeric and polymeric phthalocyanines exhibit interesting electronic, electrocatalytic and photo-electrochemical properties.
Eley and Vartanyian found in 1948 that the conductivity of phthalocyanines increases exponentially with temperature in the form of a Boltzmann distribution, which is typical for so-called intrinsic semi-conductors.
Since then there have been various publications relating to investigations of the influence of the conditions of preparation on the conductivity of monomeric and polymeric phthalocyanines. The following publications may be cited for example:
V.S. Bagotzsky et al in the Journal of Power Sources 2 (1977/78), 233-240 PA0 H. Meier et al in Berichte der Bunsengesellschaft Bd 77, nr. 10/11, 1973 PA0 H. Ziener et al: Project report to the Federal Ministry for Research and Technology, West Germany, July, 1976 PA0 M. Meier et al: Journal Physical Chemistry, 81, 712 (1977) DE OS No. 25 49 083 PA0 D. Wohrle, in Advances in Polymer Science, Vol. 10, 35 (1972). PA0 tetracyanobenzene PA0 tetracyanoethylene PA0 tetracyanopyrazine PA0 tetracyanothiopene PA0 tetracyanodiphenyl PA0 tetracyanodiphenyl ether PA0 tetracyanodiphenyl sulfone PA0 tetracyanofurane PA0 tetracyanonaphthalene PA0 tetracyanopyridine PA0 tetracyanobenzophenone.
These publications relate to formation of monomer and polymer chelates by reaction in a solution or melt. The resulting monomeric and polymeric chelates (primarily oligomers) are dissolved in concentrated sulphuric acid, diluted in water, deposited on active carbon and processed into a gas-diffusion electrode for oxygen reduction.
It has also been suggested to form polymeric phthalocyanines by a homogenous gas phase reaction of tetracyanobenzene and a volatile metal chelate, dissolution in sulphuric acid, dilution and deposition on a carbon support. This method was described for example by A. J. Appleby and M. Savy in Electrochimica Acta, Vol. 21, pages 567-574 (1976).
A. P. Berlin et al (Doklady Akademii Nauk SSR, Vol. 136, no. 5, pages 1127-1129) describe the formation of very thin films of polymeric complexes obtained from tetracyanoethylene and copper, iron or nickel. The thickness reported in the case of iron corresponded to 0.05-0.3.mu.. However, such thin films show insufficient chemical resistance in corrosive media.
Naraba et al (Japanese Journal of Applied Physics, Vol. 4 (12) 977-986, describe the preparation of a poly-tetracyanoethylene chelate film. This work relates primarily to Cu and reports a film thickness of 1 mm, with a significant Cu gradient across the film. This publication describes applying a vacuum of 10.sup.-5 mm Hg and using high frequency heating to get a clean surface; such a procedure is hardly suitable for an industrial process.
In a further publication of K. Hiratsuka et al in Chemistry Letters, pages 751-754, 1979, surface annealing under a hydrogen atmosphere is described as a prerequisite for complete removal of surface oxides prior to chelation. The temperature range of 250.degree.-350.degree. C. and an initial reactant amount related to sample area corresponding to 20-40 g/m.sup.2 are mentioned.
Polymeric phthalocyanines can exhibit high electrical conductivities which may be greater by ten orders of magnitude than the conductivities of monomeric phthalocyanines. They may have semi-conducting properties of the n or p type, depending on the conditions of preparation.
N.sub.4 -chelates and more particularly metal phthalocyanines were found to exhibit interesting catalytic properties for oxygen reduction in fuel cells where acid electrolytes are used to avoid carbonate formation.
Polymeric phthalocyanines of high molecular weight are resistant to attack by acid media and exhibit high catalytic activity for oxygen reduction.
Polymeric phthalocyanines cannot be sublimated, but it has been reported that polymeric films may be obtained after prolonged exposure of metal plates to tetracyanoethylene (TCNE) at elevated temperatures.
However, investigations have shown different methods and conditions of preparation can lead to N.sub.4 -chelates with quite different electrical and catalytic properties, as well as different molecular weights and chemical or physical stability.
It has also been found that the chemical and physical stability of oligomeric and polymeric N.sub.4 -chelates depends on the starting materials of the chelates, their purity, the conditions under which they are produced and the structure of the resulting chelate.
Thus, in spite of the evident potential interest which N.sub.4 -chelates present, their manufacture so as to provide useful industrial products is particularly difficult to achieve in a reproducible manner.
The manufacture of electrodes consisting of N.sub.4 -chelates has thus not been successfully achieved until now due to the problems of manufacturing satisfactory N.sub.4 -chelates under controlled conditions on an industrial scale.
The use of N.sub.4 -chelates as a coating material on a suitable electrically conducting substrate can provide electrodes of different shapes. However, in that case the electrode properties will also depend on the substrate material.
Proper selection of the substrate and chelate forming organic materials is thus important, in addition to suitable manufacturing conditions for the industrial production of electrodes with stable, reproducible performance.
The selected materials must be mutually compatible and also suitable for processing into stable electrodes.
A chelate coating must moreover meet the requirement of satisfactory adherence to the underlying electrode body providing a coating substrate.
Chelates with different central metal atoms can provide different catalytic properties and the selection of chelates for use as electrocatalytic materials must be made according to the intended use in each case.
In order to be able to ensure satisfactory stable performance of electrodes comprising chelates as an electrocatalytic material, loss of metal from the chelate, as well as any other degradation of the chelate by chemical or physical attacks under the operating conditions of the electrode should moreover be avoided as far as possible in each case.
The industrial processing of chelates for the manufacture of electrodes thus presents numerous problems with regard to the proper selection of electrode materials and manufacturing conditions, so as to be able to obtain electrodes with reproducible, satisfactory long-term performance which meet the high technical requirements in each case.
The state of the art relating to electrodes comprising phthalocyanines may be illustrated by U.S. Pat. Nos. 3,585,079 and 4,179,350.